1. Field of the Invention
The present invention relates to a cold cathode tube lighting device. More particularly, the present invention relates to a cold cathode tube lighting device provided with a plurality of discharge tubes connected in parallel.
2. Description of the Related Art
Cold cathode tube lighting devices have conventionally been used as light sources for various devices. As conventional examples, there are known cold cathode tube lighting devices that can be used as light sources (backlights) for liquid crystal display devices.
The discharge tube of the conventional cold cathode tube lighting device is, in terms of an equivalent circuit, a resistor whose resistance decreases non-linearly as current increases and has a non-linear negative impedance characteristic like the V-I characteristic shown in FIG. 4. Thus, when an attempt is made to drive a plurality of discharge tubes connected in parallel, there arises the following problem. That is, when an attempt is made to drive a plurality of discharge tubes connected in parallel, after the voltage across one predetermined discharge tube reaches the withstand voltage (the voltage that causes insulation breakdown), the voltage across that one predetermined discharge tube decreases owing to the non-linear negative impedance characteristic. Here, voltages across the other discharge tubes are equal to the voltage across the one predetermined discharge tube, and thus the voltages across the other discharge tubes do not reach the withstand voltage. This makes it difficult to light all of the plurality of discharge tubes.
One possible way to solve the problem just described is to connect separate inverter power supplies to each one of the plurality of discharge tubes. This, however, leads to disadvantages such as increased size and cost of cold cathode tube lighting devices.
To cope with this, cold cathode tube lighting devices provided with a discharge tube having a ballast capacitor connected thereto have conventionally been proposed (for example, see JP-A-H10-177170). The cold cathode tube lighting device according to JP-A-H10-177170, in terms of an equivalent circuit, has a capacitor connected to a resistor whose resistance non-linearly decreases with increase in current, and thus has a non-linear positive impedance characteristic like the V-I characteristic shown in FIG. 5. Thus, according to JP-A-H10-177170, when a plurality of discharge tubes connected in parallel are driven, all of the plurality of discharge tubes can be lit.
With the conventional cold cathode tube lighting device provided with a plurality of discharge tubes connected in parallel, even when a failure occurs in any of the plurality of discharge tubes, the lighting operation of the cold cathode tube lighting device continues to be performed without stopping if the other discharge tubes are operating normally. Thus, the conventional cold cathode tube lighting device is inconvenient in that it continues its lighting operation in a state in which there exist one or more discharge tubes that are unlit or degraded in brightness. This leads to a problem of emission-position-dependent unevenness occurring in the brightness of light emitted from the cold cathode tube lighting device.
Furthermore, when the conventional cold cathode tube lighting device is used as a backlight for a liquid crystal display device, emission-position-dependent unevenness occurring in the brightness of light emitted from the cold cathode tube lighting device leads to an inconvenience of degraded display quality of the liquid crystal display device.